Video teleconferencing for networked workstations

ABSTRACT

A video teleconferencing method and apparatus for computer workstations connected by a digital data network includes a transmission source portion for a local workstation to send audio and video teleconference data across the network to one or more remote workstations, and, a receiver for the local workstation to receive audio and video teleconference data back from the remote workstations. The local workstation sends teleconference data to each of the remote workstations over a variable bandwidth digital data connection, and each of the remote workstations returns teleconference data back to the local workstation over another variable bandwidth digital data connection. The transmission source portion includes a master software process executing on the local workstation, and the receiver includes a slave software process executing the remote workstation. The master process of a local workstation causes execution of a slave process on a remote workstation for receiving video teleconference data from the local workstation. An audio data transmitter for sends an audio data stream to the remote workstation such that the audio data can be reconstructed into a continuous audio signal. A video transmitter sends video data so that each frame of video data to be sent is inserted into the audio data stream without affecting the continuity of the reconstructed audio signal at the remote workstation.

BACKGROUND OF THE INVENTION

Video communications has evolved over the years from a simple videotelephone concept to a sophisticated network for allowing multiple usersto participate in a video teleconference. For full featured videoteleconferencing, users require both an audio communications path and areal time visual communication path synchronized to the audio path.Furthermore, it is desirable to support full color video and telephonequality audio. Video teleconferencing capabilities are limited mainly bythe bandwidth of the transmission medium connecting the teleconferencingterminals.

Many computer workstations used in the office or laboratory environmenttoday are connected with other workstations, file servers, or otherresources over high-speed local area networks. Local area networks, inturn, are often connected together through high-speed speed gatewayswhich connect workstations which may be distributed over a widegeographic area. Network wide protocols allow workstations to exchangepackets of data at high rates of speed and reliability. Fixed bandwidthdigital and analog video channels have been combined with computernetworks to implement some video teleconferencing features. Theseinclude high bandwidth CATV/FDM type analog channels and fixedallocation TDM data channels for the video data.

SUMMARY OF THE INVENTION

Workstations today have obtained unprecedented computational power andutility. The powerful RISC type CPUs and fast, high resolution graphicaldisplays have made possible multimedia workstations which integrate liveaudio and video into the programming environment. Graphical UserInterface operating systems (GUI) have allowed effective integration ofaudio and video into application programming.

The present invention provides n-way video teleconferencing amongnetworked computer workstations using the existing variable bandwidthdigital data network for transferring synchronized audio and videoteleconferencing data between the workstations. The teleconferencingapparatus and protocol of this invention provides high quality videoteleconferencing without the need for a guaranteed wide bandwidth analogvideo channel or a fixed allocation digital video channel. Rather, theinvention uses standard non-allocated data packets typically found onlocal area networks to transfer the audio and video teleconferencingdata. Thus, no guaranteed bandwidth is required to carry on a usefulvideo teleconference. An continuous audio data stream model providescontinuous audio signals at the expense of video data when necessary,which is desirable since the ear is more sensitive to a break in theaudio data than the eye is to the loss of a frame of video data. A"push" data model provides a secure system by preventing remoteworkstations from activating another workstation's videoteleconferencing functions.

In general, in one aspect, the invention features a videoteleconferencing method and apparatus for computer workstationsconnected by a digital data network. The computer workstations include atransmission source means for a local workstation to send audio andvideo teleconference data across the network to one or more remoteworkstations, and, a receiver for the local workstation to receive audioand video teleconference data back from the remote workstations. Thelocal workstation sends teleconference data to each of the remoteworkstations over a variable bandwidth digital data connection, and eachof the remote workstations returns teleconference data back to the localworkstation over another variable bandwidth digital data connection. Thevariable bandwidth digital data connections include the data packetoriented data channels associated with, for example, FDDI, DECnet, andEthernet local area networks. Furthermore, a wide area digital network,such as ISDN, can also be used with the video teleconferencing apparatusand method of this invention.

In preferred embodiments, the transmission source means includes amaster software process executing on the local workstation, and thereceiver includes a slave software process executing the remoteworkstation. The master software process formats and sends videoteleconference data to the slave process. The slave process receives andreconstructs the audio and video teleconference data for audible andvisual reproduction, respectively. The video data is presented as animage on the display of the receiving workstation, while the audio datais sent to either amplified speakers or headphones. In other preferredembodiments, the master process of a local workstation causes executionof a slave process on a remote workstation for receiving videoteleconference data from the local workstation. The slave processrunning on the remote workstation in turn causes execution of a masterprocess on the remote workstation for sending video teleconference databack to the local workstation. The master process of the remoteworkstation in turn causes execution of a slave process on the localworkstation for receiving the video teleconference data sent by themaster process of the remote workstation. The local workstation executesa slave process for each master process on a remote workstation sendingvideo teleconference data to the local workstation.

In yet other preferred embodiments, the transmission source includes anaudio data transmitter for sending an audio data stream to the remoteworkstation such that the audio data can be reconstructed into acontinuous audio signal. The transmission source also includes a videotransmitter for sending video data to the remote workstation so thateach frame of video data to be sent is inserted into the audio datastream without affecting the continuity of the reconstructed audiosignal at the remote workstation.

In yet other preferred embodiments the video transmitter precludes aframe of video data from being sent to the remote workstation if asystem overload exists. The audio transmitter sends the audio datastream corresponding to the precluded video frame to the remoteworkstation to prevent loss of continuity of the audio signal during asystem overload. In other preferred embodiments the video transmitterprecludes a frame of video data from being sent to the remoteworkstation in response to a system failure condition. The audiotransmitter accumulates audio data for a predetermined time intervalduring the system failure condition, and transmits the accumulated audiodata stream to the remote workstation once the failure has beencorrected. For instance, the audio transmitter may continuallyaccumulate the last 1/2 second of audio data while the failure exists,trimming any audio data older than 1/2 second. The last 1/2 second ofaudio data accumulated before the failure's correction is sent to theremote workstation as soon as the failure is corrected. The mostrecently available frame of video is then also sent.

In still other preferred embodiments, timing information is attached toeach frame of video data sent to the remote workstation. The timinginformation indicates a point in the continuous audio data stream whichcorresponds in time to the frame of video data. The receiver of theremote workstation includes a synchronizer for displaying a receivedframe of video when the point in the audio stream corresponding to thetiming information of the received video frame is audibly reproduced atthe remote workstation. The synchronizer counts the amount of audio datareceived in the continuous audio stream and compares the count to thetiming information sent along with the most recently received videoframe to determine when to display the frame.

In general, in another aspect, the invention features a multimediacomputer workstation, such as a RISC workstation or IBM PC, having videoteleconferencing capabilities. The multimedia workstation of thisinvention includes a network interface for establishing a variablebandwidth digital communications channel across a digital data networkwith another multimedia workstation. A video source provides a frame ofdigitized video data, and an audio source provides digitized audio dataassociated with the frame of video data. A data transmitter transmitsthe audio and video data through the network interface across thevariable bandwidth digital communications channel to anotherworkstation. A receiver receives audio and video data through thenetwork interface across the variable bandwidth digital communicationschannel from another workstation. The workstation also includes meansfor displaying the received video data on the workstation display, andmeans for audibly reproducing the received audio data.

In preferred embodiments, the video source includes a video camera, avideo tape recorder, and/or a video laser disk player providing framesof analog video. A video frame grabber captures, digitizes, and storeseach frame of analog video. The video source also includes digital videodata stored in a file accessible by the workstation. A video compressormay compress the video data using JPEG or MPEG compression. The audiosource includes a microphone for live audio, or pre-recorded audiocorresponding to frames of pre-recorded video, from for instance a videotape recorder or laser disk. An audio digitizer digitizes and stores theaudio using mu-law compression. The audio source also includes digitalaudio data stored in a file, preferably along with digital video data,accessible by the workstation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a pictorial representation of the distributed computer systemfeaturing multimedia workstations having video teleconferencingcapabilities of this invention.

FIG. 2 is a block diagram showing a preferred embodiment of themultimedia workstation for implementing the video teleconferencingfeatures of the distributor computer system of FIG. 1.

FIG. 3 is a block diagram showing the software environment forimplementing a preferred embodiment of a DECspin video teleconferencingapplication for implementing the video teleconferencing features of thesystem of FIG. 1.

FIG. 4 is a block diagram showing the one-way connections establishedbetween two workstations to implement a two-way video teleconference.

FIGS. 5a-5c are block diagrams illustrating the multiple one-way videoteleconferencing connections established between three workstationsjoined in the same teleconference.

FIG. 6 is a flowchart illustrating the flow of video during a videoteleconference.

FIG. 7 is a flowchart illustrating the teleconferencing protocol of thisinvention for establishing a video teleconference connection between twoworkstations.

FIG. 8 illustrates the format of the START message of theteleconferencing protocol of this invention.

FIG. 9 illustrates the format of the OK message of the teleconferencingprotocol of this invention.

FIG. 10 illustrates the format of the STARTHEADER message of the videoteleconferencing protocol of this invention.

FIG. 11 illustrates the format of the QUIT message of theteleconferencing protocol of this invention.

FIG. 12 illustrates the format of the ANSWERMACHINE message of theteleconferencing protocol of this invention.

FIG. 13 illustrates the format of the CONTROL message of the videoteleconferencing protocol of this invention.

FIG. 14 is a flowchart showing the audio and video data transfersequence of the video teleconferencing protocol of this invention.

FIG. 15 shows the format of the AUDIOHEADER message of the videoteleconferencing protocol of this invention,

FIG. 16 shows the format of the VIDEOHEADER message of the videoteleconferencing protocol of this invention,

FIG. 17 shows a timing diagram of the audio and video datasynchronization of the video teleconferencing protocol of thisinvention.

FIG. 18 shows the top level graphical user interface window forcontrolling a video teleconferencing session of this invention.

FIG. 19 shows the graphical user interface window for displaying videodata received from another workstation during a video teleconferencingsession of this invention.

FIG. 20 shows a second level graphical user call list interface windowfor establishing the video teleconferencing connections to otherworkstations of this invention.

FIG. 21 shows a second level control graphical user interface window foradjusting the parameters for a video teleconference of this invention.

FIG. 22 shows a second level monitor graphical user interface window formonitoring the parameters of a video teleconference of this invention.

FIG. 23 shows a second level documentation graphical user interfacewindow for obtaining information about the operation and features of avideo teleconference of this invention.

FIG. 24 shows a third level documentation graphical user interfacewindow for obtaining information about a topic selected from the secondlevel user interface window of FIG. 23.

FIG. 25 shows a graphical user interface window of a ring box forannouncing a video teleconference call to another workstation toestablish a video teleconference of this invention.

FIGS. 26(a)-26(k) show the dislay screens of three Workstationsparticipating in a three-way video teleconference of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a distributed computer system 10, having a plurality ofmultimedia workstations 12 connected by a high speed digital network 14,which features n-way video teleconferencing of this invention. Each ofthe multimedia workstations 12 is capable of producing live audio andvideo data for transmission across the network to another multimediaworkstation. Further, each of the multimedia workstations is capable ofsonically reproducing the audio data and visually reproducing the videodata received from another workstation.

Two or more of the networked workstations can participate in an n-wayvideo teleconference using the teleconferencing protocol of thisinvention. The teleconferencing protocol of this invention allows realtime synchronized audio and video transmission across the networkwithout the use of a fixed bandwidth or dedicated time slot transmissionmedium. Rather, this invention provides high performance videoteleconferencing features using standard digital network transport levelprotocols such as Internet TCP/IP and UDP/IP, or DECnet™. The physicalnetwork link 14 should be a high speed FDDI (Fiber Distributed DataInterface) fiber optic link running at 100 MB/Sec. for maximumperformance (about 15 uncompressed black and white frames per second),but can also be virtually any type of high quality digital network linksuch as an Ethernet™. In the case of the FDDI network link, performanceis typically limited by the workstation hardware and software, ratherthan by the throughput of the network. In addition, wide area networking(WAN) technologies, such as T1 and T3 digital carriers, can be used withthe video teleconferencing protocol of this invention. These WANtechnologies can be expected to achieve a video frame rate of at leastabout 12 Hz, assuming black and white video images compressed with JPEG,MPEG, or another video compression technique. The features of onepreferred embodiment of this invention are commercially embodied in theDECspin™ product available from Digital Equipment Corporation, Maynard,Mass., the assignee of this patent application.

FIG. 2 shows a preferred embodiment of the multimedia workstation 12 forimplementing the video teleconferencing protocol of this invention.Workstation 12 includes a high performance processor 20 connected to alarge working memory 22 having 24 megabytes or more capacity, and alarge hard drive having 1 gigabytes or more capacity. A high performancebackplane channel 26 connects the processor, memory, and hard disk toother devices in the workstation.

The workstation is coupled to the digital network communications link 14through a network controller 28 connected between the network link 14and the backplane channel 26. The workstation is also coupled to a voicegrade telephone line 29 through a modem 31 connected between thebackplane channel and the telephone line. Similarly, the workstation canbe connected to other digital data conversation services, such as theISDN digital telephone network.

Multimedia workstation 12 includes a color video frame grabber 34 forcapturing and digitizing frames of video from one or more analog videoinputs 36. Video inputs 36 are each connected to a video source, such asa video camera 38 providing live analog video signals, or an auxiliaryvideo storage device 40, such as a VCR or video laser disk playerproviding stored analog video signals. The analog video signals may beof any standard types such as NTSC, SECAM, or PAL.

The multimedia workstation 12 also includes a video buffer 35 whichstores a frame of full color video graphics and characters generated bythe workstation for display on a 1280×1024 pixel color monitor 30. Thevideo buffer 35 (and associated buffer controller) is connected to thebackplane channel 26 for receiving video data from the processor 20. Thevideo buffer is also connected to a video display subsystem 36 whichconverts the stored video frame data into analog signals for driving thecolor monitor 30.

The video frame grabber 34 stores its digitized video data directly intoa predetermined area of the video buffer 35. Thus, the digitized videoinput to the workstation by the frame grabber appears directly in apredetermined area on the monitor 30, without having to pass throughprocessor 20 or main memory 22. Further, processor 20 can read back thecaptured video frame data from the video buffer, store the data in mainmemory 22, and further process the video data according to the videoteleconferencing protocol described herein.

Frame grabber 34 digitizes and stores each frame of video from an analogvideo source and can deliver up to 30 frames per second of digitized640×480 true color (24 bits) of NTSC/SECAM/PAL video into video framebuffer 35. A dedicated hardware video compression subsystem 37 can alsobe connected to the backplane channel 26 to provide high performancevideo compression of the digitized video data.

The audio features of the multimedia workstation 12 are implementedusing an audio controller 42 connected to the backplane channel 26 forinterfacing audio signals into the workstation and reproducing audiosignals out of the workstation. An audio distribution box 44 isconnected to the audio controller for directing audio to and from audiodevices such as a microphone 46, a headphone 48, and/or a telephonereceiver 50. Auxiliary audio devices such as a recording device, a CDplayer, or amplified speakers may also be connected to the distributionbox 44. Audio controller 42 receives audio input from the distributionbox and digitizes the audio using an 8 bit mu-law encoder at 64 kbitsper second or less to match typical telephone voice grade audiostandards (8 kHz bandwidth). For convenience, the headphones 48 andmicrophone 46 may be combined into a single headset.

In one commercial embodiment available from Digital EquipmentCorporation, multimedia workstation 12 is built around a DECstation™5000/200 workstation in which processor 20 is an R3000™ RISC processorand backplane channel 26 is a TurboChannel™ (TC) bus. Network controller28 is a DEC FDDIcontroller 700™. Frame grabber 34/video buffer 35/videodisplay subsystem 36 is a DECvideo/in™ TurboChannel compatible videosystem, and audio controller 42 is a DECaudio™ TurboChannel compatibleaudio controller.

Alternatively, multimedia workstation 12 can be built around a personcomputer platform, such as an IBM™ compatible PC. Preferably, the PCshould have a high speed Intel 80386™, 80486™, or successor processor.The PC should be compatible of running in the Microsoft Windows™ orWindows NT™ graphical operating system environment.

Workstations and PCs of different performance levels can all participatein the same video teleconference using the video teleconferencingprotocol of this invention. For instance, less powerful or slower PC'sand workstations can simply communicate at a slower video rate with themore powerful workstations, without affecting the overall video ratebetween other workstations.

The effective video frame rate of a video teleconference depends to alarge extent on the quantity of video data handled by a workstationduring each video frame. Video compression techniques can greatlyenhance the effective frame rate of the video teleconference especiallyover lower data transfer rate services such as ISDN. Standardcompression methods such as JPEG (Joint Photographic Experts Group),MPEG (Motion Picture Experts Group) and P×64 can be implemented using adedicated hardware subsystem 37 which have recently become available.

A video compression technique which has been found useful for use withthe video teleconferencing protocol of this invention features pixeldecimation and replication (PDR). This PDR technique effectivelysubsamples by 1/2 a full frame of video pixels in the vertical (Y) andhorizontal (X) directions. Thus, a frame of video captured by the framegrabber is reduced to one quarter of its original size. The subsamplingis accomplished by a scaling function featured on the frame grabber 34of FIG. 2.

Reconstruction of the image is implemented in software by replicatingevery pixel received twice along a line, and then replicating thereconstructed line as the next line of the restored image. Thereplicated line is shifted by a single pixel to prevent any pixel blockpatterns from forming. Results have shown acceptable quality of thereceived image, while offering significantly increased system throughputand performance.

FIG. 3 shows a diagram of the DECspin™ product software systemenvironment for a preferred embodiment of this invention. Generally, theDigital Teleconferencing Protocol (DTP) is implemented by a DECspin™application program 60 which resides at the application layer of thestandard ISO 7-layer network model. The DTP video teleconferencingprotocol of this invention is used to transfer audio and video databetween workstations, as well as between local applications (loopback),and for storing and retrieving audio and video data to and from a diskfile.

The DECspin application program 60 runs on top of an Ultrix™ (V4.2a orits successors) (UNIX™) 62 operating system. All communications betweenthe DECspin application 60 and the UNIX operating system 62 are handledvia UNIX system calls 64. The DECspin application program 60 is writtenas an X Windows™ application running under a Motif™ X Windows manager. Adescription of the Ultrix, X Windows, and X11 programming environmentscan be found in "ULTRIX Worksystem Software, X Window System Protocol: XVersion 11," Digital Equipment Corporation order , number AA-MA98A-TE,Maynard, Mass. (1988, Version 2.0), the contents of which areincorporated herein by reference. A description of the Motif programmingenvironment can be found in Open Software Foundation, "OSF/MotifProgrammer's Guide," Prentice Hall, Englewood Cliffs, N. J. 07632 (1991,Revision 1.1), the contents of which are incorporated herein byreference.

Through UNIX system calls the DECspin application establishes localTCP/IP "sockets" for communication with various software serversproviding multimedia services for the DECspin application. The creationand use of UNIX "socket" system calls for creating TCP/IP applicationconnections is discussed in detail by Comer, D. E., "InternetworkingWith TCP/IP, Vol. 1: Principles, Protocols, and Architecture, SecondEdition" Prentice Hall, Englewood Cliffs, N.J. (1991) (pp 337-346), thecontents of which is incorporated herein by reference. The DECspinapplication communicates with an audio server application 66 whichprovides audio services using X Windows Xmedia commands 68 through alocal TCP/IP "socket". The DECspin application communicates with a videoX Windows Xserver application 70 which provides video services using X11and XV extension commands 72 through another local TCP/IP "socket".DECspin communicates with a Motif based graphical user interface (GUI)74 through yet another local TCP/IP "socket".

The DECspin application communicates with a DECspind "slave" application78, running on either the local or a remote workstation using the DTPvideo teleconferencing protocol 80 via another "socket". The DECspindapplication is a UNIX "deamon" version of the DECspin application whichis "slaved" to a DECspin "master" application running on either thelocal or a remote workstation. The DECspind application is used toreceive, reconstruct, synchronize, and display audio and video data sentto it from either a local or remote DECspin application using the DTPprotocol. Where the DECspind application is operating on the localworkstation, the "socket" is a local TCP/IP "socket". Where the DECspindapplication is operating on a remote workstation, the "socket" can beeither a remote TCP/IP or DECnet "socket".

Furthermore, the DECspin application can store audio and video data to asystem file 82 using the DTP video teleconferencing protocol 84 viaanother "socket". This creates a stored "film clip" audio and video datastream which can be played back using a DECspind application. That is, aDECspind application creates a "socket" to the stored file, and receivesdata from through the socket via the same DTP protocol. This is theequivalent of the DECspind application receiving the audio and videodata directly from a DECspin application via the DTP videoteleconferencing protocol, but without the acknowledgement protocoldiscussed below.

FIG. 4 shows a schematic model of two workstations 12a and 12b connectedacross network 14 during a two-way video teleconference run under theDECspin application. Each "two-way" video teleconferencing connectionestablished between two workstations is composed of separate "one-way"connections. Each one-way connection operates in a "data pushing" mode.That is, audio and video data are "pushed" from the originatingworkstation to the receiving workstation. This "push" model provides ahigh degree of security since it prevents another network user fromremotely activating workstation's audio and video from across thenetwork.

To originate a video teleconference call to another workstation, a firstworkstation 12a (Workstation A) executes a local DECspin applicationprogram 100 which acts as a "master". DECspin "master" 100 firstestablishes a one-way network connection 102 with a second workstation12b (Workstation B) by invoking execution of a DECspind "slave"application program 104 on Workstation B. Digitized audio and video dataare sent from Workstation A by DECspin application 100 to Workstation Bwhere they are received by the DECspind application 104. Furthermore,DECspind application 104 signals 106 the user of Workstation B toindicate that a video teleconference call has been received fromWorkstation A. In response to the signal, a local DECspin "master"application program 108 on Workstation B establishes a separate one-waynetwork connection 110 with Workstation A by invoking execution of aDECspind "slave" application program 112 on Workstation A. If theDECspin application 108 on Workstation B is not currently executing whenthe signals 106 occur, the DECspin application 108 is executed andplaces a return call to Workstation A. The return call to Workstation Acauses the execution of DECspind application program 112. Audio andvideo signals from Workstation B are sent by DECspin application 108 toWorkstation A where they are received by DECspind application 112 tocomplete the two-way video teleconferencing connection between theworkstations.

FIG. 5a symbolically shows the two one-way video teleconferencingconnections of FIG. 4 between Workstation A and Workstation B. FIGS. 5band 5c symbolically show the addition of a third workstation 12c(Workstation C) to the video teleconference. As shown in FIG. 5b,Workstation A first establishes a video teleconference with WorkstationC independent of Workstation A's video teleconference with WorkstationB. This new video teleconference is invoked by Workstation A in a manneridentical to the invocation of the conference between Workstation A andWorkstation B as shown in FIG. 4, i.e., by creating two one-wayconnections 102a and 110a between the workstations using DECspin andDECspind application programs running on the two workstations.

Once a video teleconference between Workstation A and Workstation C isestablished, Workstation C and Workstation B likewise establish twoone-way connections 102b and 110b as shown in FIG. 5c to complete thevideo teleconferencing connections. This can occur by either WorkstationB calling Workstation C, or by Workstation A "joining" all theworkstations into the same conference by sending a "join" controlmessage to each of the other workstations. Upon receipt of the "join"message, each workstation places a call to each other, unconnectedworkstations. Thus, each workstation participating in a particular videoteleconference establishes and maintains a two-way video teleconferenceconnection with each other workstation in the teleconference. In thismanner, a teleconference participant can control what each otherparticipant receives from that workstation, e.g., muting audio orpausing video to certain participants while remaining active to otherparticipants. Furthermore, if a participant has a workstation which canonly operate at a limited capacity, each participant can treat thatworkstation differently from the other workstations, without affectingthe overall performance of the video teleconference for the otherparticipants. For example, an audio only workstation can stillparticipate in a teleconference without affecting the video distributionamong the other workstations.

It should be noted that each workstation will run only a single DECspinapplication, which may establish a connection with one or more remoteworkstations. However, each workstation will execute one DECspind"slave" application session for each workstation to which a videoteleconference connection is established. That is, each workstation runsa single DECspin "Master" application, but may run multiple DECspind"Slave" applications simultaneously.

In one preferred embodiment, a single "master" DECspin application canbe connected to up to seven DECspind "slave" applicationssimultaneously. Further, all the DECspind applications need not bemembers of the same video teleconference. For instance, one workstationmay simultaneously be connected to two separate video teleconferences,one with three members and one with four members, for a total of sevenDECspind applications driven from a single DECspin application. Themembers of each conference can only see and hear the members of the sameconference. Any workstation common to both conferences can "join" theconferences, in which case all seven members will be joined into asingle conference where all can see and hear each other.

FIG. 6 is a block diagram showing the flow of video from one workstationto another. The video data source (38 or 40 of FIG. 2) provides standardNTSC, SECAM, or PAL analog video signals 200 which are digitized andstored 202 in a video frame buffer 35 by the frame grabber 34 (FIG. 2).Once an entire video frame has been digitized and stored in the framebuffer, the entire frame of video data is transferred 204 to anapplication buffer in main memory 22 (FIG. 2) which is assigned to theDECspin application. Once the video data has been transferred to theapplication buffer, the video frame grabber can begin to digitize andstore the next video frame in the frame buffer.

The digitized video data in the DECspin application buffer is nextpackaged for decoding and reconstruction by the receiving DECspindapplication, and is sent to the network transport buffer 206. The videodata in the network buffer are packetized according to the networkprotocol in use, e.g., TCP/IP or DECnet, sent 208 across the network,and received 210 into a receiving network buffer. The video data isreconstructed from the network protocol packets and sent 212 to aDECspind application buffer in main memory 22 (FIG. 2) of the receivingworkstation. The video data are accumulated into the DECspindapplication buffer until a full frame of video has been received.Graphics are added to the video to form a composite video image fordisplay. The composite video image is then sent 214 to the frame bufferfrom which the digitized composite video image is converted for display216 on the workstation monitor.

FIG. 7 is a flowchart showing the commencement of a video teleconferencebetween a local workstation, Workstation A, and remote workstation,Workstation B. To begin a teleconference, the user of Workstation Ainvokes 300 the execution of the local DECspin application. Theexecuting DECspin application causes the creation of a network "socket"302 through a standard UNIX system call. Creation of the socket onworkstation A causes the creation of a corresponding "port" 304 on theremote Workstation B. This "socket-to-port" connection establishes apeer-to-peer connection for the DECspin and DECspind applicationsrunning on the two-connected workstations. That is, data put into the"socket" on Workstation A are transferred to the port on Workstation Bvia a standard network protocol such as TCP/IP or DECnet. UNIX treatsthe established socket as a standard file descriptor to which theDECspin application simply sends the data stream to be transferred toWorkstation B. Thus, this data transfer operation is transparent to theDECspin application which simply accesses the "socket" through standardUNIX calls.

Once the "socket" to Workstation B is established, DECspin sends a STARTmessage 306 through the "socket" to Workstation B. FIG. 8 shows theformat of the START message, which is simply a two byte long datastring.

Upon receipt of the START message, Workstation B invokes execution 308of a DECspind "slave" application which connects to the previouslycreated port 304 to receive data from the "socket" on Workstation A.Once the DECspind application verifies 310 this end-to-end connectionbetween the two workstations, DECspind sends 312 an OK acknowledgementmessage through the "port" back to the DECspin application. FIG. 9 showsthe format of the OK message, which is simply a two byte data stream.

Upon receipt of the OK message from the "socket," the DECspinapplication sends 316 a STARTHEADER message through the "socket" to theDECspind application which is used to set up 318 the parameters for thecurrent video teleconferencing session. FIG. 10 shows the format of theSTARTHEADER message, the fields of which convey the parameterinformation required by the DECspind application. These parameter fieldsinclude a video width field 400 and a video height field 402 for settingthe initial DECspind picture size. A bits per pixel field 404 setsDECspind to display either a black and white image (8 bits) or a truecolor image (24 bits). A frame rate field 406 sets the desired initialframe rate. A frame count field 408 sets the number of frames expectedto be sent during the video teleconference. The value of this field isset to -1 for a live video teleconference (to indicate infinity) wherethe total number of frames to be sent is indefinite. Otherwise, theactual number of frames to be transferred during the session is enteredinto the field, e.g., for a video message of predetermined duration. Alogin/user handle field 410 identifies the calling party and is used byDECspind to set up the return audio and video connections fromWorkstation B to Workstation A. Finally, a DTP (DECspin teleconferenceprotocol) flag field 412 conveys multiple single bit flags whichindicate particular operational modes such as video compression typeused (if any) audio encoding type used, whether to expect audio, video,or both, or how DECspind should respond to certain commands.

Once DECspind receives the STARTHEADER and determines 320 that it willparticipate in the video teleconference with Workstation A, it issuesanother-0K message to the DECspin application on Workstation A. Thereceipt 322 of this OK message causes the DECspin application to begintransmission 324 of audio and video data to the DECspind application onWorkstation B.

Furthermore, when the user answers the call the DECspind applicationinvokes execution of its own local DECspin application, whichestablishes a return connection for sending return audio and video datafrom Workstation B to Workstation A, to complete the two-way videoteleconference. The DECspin "master" application now running onWorkstation B establishes communications with a DECspind "slave"application on Workstation A in a process equivalent to that justdescribed with respect to establishing communications betweenworkstation A and workstation B. In this case, however, Workstation B isviewed as the local workstation, and Workstation A is viewed as theremote workstation.

With respect to the invocation 326 of the DECspin application onWorkstation B, this may occur in one of two ways. If there is no DECspinapplication currently running on Workstation B, then DECspind initiatesexecution of the DECspin application, and passes the required parametersto the DECspin application through the application invocation commandline. If the DECspin application is already running on Workstation B(e.g., supporting another video teleconference in progress), theDECspind application will append the parameters received via theSTARTHEADER message into an information file used by the executingDECspin application. This information file is in turn used to controlexecution of the DECspin application. Only one DECspin application maybe executing on a workstation at one time. However, multiple DECspindapplications may be running on the same workstation.

Alternatively, if the DECspind application running on Workstation Breceives the STARTHEADER message and does not want to, or cannotparticipate in the video teleconference, it returns 228 a QUIT messageto the DECspin application on Workstation A. FIG. 11 shows the format ofthe QUIT message, which is a two byte data string. Upon receipt of theQUIT message, the DECspin application on Workstation A terminates theconnection with the DECspind application on Workstation B.

As another alternative, the DECspind application running on WorkstationB can respond to the STARTHEADER like a telephone answering workstationby returning an ANSWERMACHINE message back to the DECspin application.FIG. 12 shows the format of the ANSWERMACHINE message which is also atwo byte data string. The DECspin application running on Workstation Aallows the user to respond to the ANSWERMACHINE message by sending afixed length (typically 20 seconds) audio and video message to theDECspind application on Workstation B where it is stored in a file fordeferred playback by the user. The audio and video message may also beviewed by the user of workstation B as it is arriving for storage,similar to "screening" an incoming telephone call.

FIG. 13 shows the format of a CONTROL message, the fields of whichconvey control information and commands between workstationsparticipating in a video teleconference. Specifically, a two byteidentifier field 450 identifies the CONTROL message. At which field 452identifies the type of control message to follow, and a length field 454indicates the length of the control message. A flag field 456 conveysother information about the CONTROL message to the receivingworkstation, for instance whether or not an acknowledgement messageshould be sent to the originator of the CONTROL message upon itsreceipt.

One type of CONTROL message is a "JOIN" message. This message is sent bya local workstation to all remote workstations currently participatingin a video teleconference with the local workstation to join all theworkstations into the same video teleconference. As discussed above, aworkstation may carry on more than one video teleconferencesimultaneously without the other participants being connected to eachother. Upon receipt of a "JOIN" CONTROL message, each of the remoteworkstations places a video teleconference call to each of the otherremote workstations which are currently connected to the localworkstation but not to that remote workstation. In this manner,individual dual one-way video teleconferencing connections areestablished between each of the workstations that has been "joined" intothe teleconference. It should also be noted that any. workstation canleave the teleconference without affecting the teleconference among theremoving workstations by simply terminating its video teleconferenceconnection to each of the workstations.

FIG. 14 is a flowchart showing the synchronized transmission ofdigitized audio and video data from a DECspin application running on alocal workstation, Workstation A, to a DECspind application running on aremote workstation, Workstation B. Video data are collected on acontinuing frame by frame basis. One frame is collected and stored inthe frame buffer of the frame grabber 34 (FIG. 2) until the frame istransferred to the DECspin application buffer in main memory 22 (FIG. 2)for transmission across the network to a DECspind application, or untilthe frame is discarded to make room for collection of a new video frame.Digitized audio data are collected into an audio data buffer. The audiodata is drawn out of the audio buffer by the DECspin application whichsends the audio data as a continuous stream to the DECspind application.

The audio and video data sent across the network can be modeled asframes of video data inserted into a continuous stream of audio data. Ingeneral, the DECspin application will not discard audio data, but willdiscard video frames as necessary. Since the ear differentiates soundand the eye integrates images, breaks in the audio are moreobjectionable to a listener than breaks in the video are to a viewer. Asa result, the highest priority of the DECspin application is to delivera continuous, unbroken digitized audio stream to the DECspindapplication. 0n the other hand, video frames are routinely discardeddependent on network throughput, and other workload factors.

After a video frame has been digitized and stored in the frame grabberbuffer, the frame is made available 350 to the DECspin application fortransfer to the DECspind application running on Workstation B. Digitizedaudio stored in the audio buffer, up to the time when the frame becomesavailable, is sent 352 first to the DECspind application. The DECspinapplication begins the audio data transmission by sending an AUDIOHEADERmessage through the "socket" to the DECspind application. FIG. 15 showsthe format of the AUDIOHEADER message, the fields of which conveyinformation about the audio data to follow. Specifically, a two byteidentifier field 414 identifies the AUDIOHEADER message, a length field416 indicates the amount of digitized audio data to follow theAUDIOHEADER message, and an audio flag field 418 indicates parametricinformation about the audio data. One such audio flag, for instance,indicates the type of audio encoding used to digitize the audio signal,e.g., mu-law encoding. Packets of up to 16 kB each of audio data arethen sent to the DECspin "socket" immediately following the AUDIOHEADERmessage. The audio data packets are reassembled into a continuous audiodata stream by the receiving DECspind application, and the audio issonically reproduced.

Next, the frame of available video data 350 is sent 354 from the DECspinapplication to the DECspind application immediately following the audiodata. The DECspin application begins the video frame transmission bysending a VIDEOHEADER message through the socket to the DECspindapplication. FIG. 16 shows the format of the VIDEOHEADER message, thefields of which convey information about the frame of video data tofollow. Specifically, a two byte identifier field 420 identifies theVIDEOHEADER message. A video width field 422 and video height field 424indicate the size of the video frame to follow the VIDEOHEADER message.A control flag field 426 conveys parametric information about the videodata frame. Finally, a timing information field 428 carries a time stampwhich helps the DECspind application to realign the audio and video datafor synchronized reconstruction at workstation B. The time stamp is anumerical value corresponding to the byte count of the continuous audiostream taken at the end of the current video frame.

A full frame of digitized video data is sent from the DECspinapplication to the DECspind application immediately following theVIDEOHEADER message. The DECspin application then stops sending datathrough the "socket" 356 until it receives an OK message (FIG. 9) fromthe DECspind application which acknowledges receipt and successfulreconstruction of the full video frame.

The video frame data are broken into data packets of 32 kB or less tofacilitate efficient transmission through the "socket" and across thenetwork by the TCP/IP protocol. Although the video data could be sentthrough the "socket" as a single, unbroken stream of approximately 300kbytes of data (for a black and white image having 640 by 480 pixels,each 8 bits deep), any error occurring at the TCP/IP protocol levelwould require retransmission of the entire 300 kbyte data stream.Therefore, by breaking the data into smaller 32 kbyte maximum sizepackets, any error at the TCP/IP protocol level will only requireretransmission of at most 32 kbytes of video data. This significantlyincreases the efficiency of video transmission across the network whichresults in a higher overall video throughput.

The DECspind application collects the received video data packets intoan application buffer until an entire frame of video data has beencollected. At that time, the DECspind application proceeds to displaythe received video frame to the workstation user, and issues an OKmessage (FIG. 9) to the DECspin application to acknowledge receipt andsuccessful reconstruction of the video frame.

The DECspin application responds to the OK message sent by the DECspindapplication in one of three ways. The response depends on when the OKmessage is received relative to the time required for the video framegrabber to make the next video frame available, i.e., "the frame time"between available frames. FIG. 17 illustrates the timing and operationsassociated with the receipt of the OK message by the DECspinapplication, and is discussed in detail below.

The first situation occurs when DECspin receives the OK message withintwo frame times 358 (FIG. 14) of sending the last available video frameto the DECspind application. In this case, DECspin returns 360 todetermine if 350 a new video frame is available from the frame grabberbuffer, and the entire process of sending audio and video data to theDECspind application is repeated.

The second situation occurs when DECspin receives the OK message beyondtwo frames times 362, but less than a "time out" time period, aftersending the last available video frame to the DECspind application. Inthis case, DECspin first sends 364 the digitized audio accumulated inthe audio buffer to the DECspind application to prevent loss of thisaudio data which would result in a break in the continuity of thereconstructed audio. DECspin then proceeds to return 360 and determine350 if a new video frame is available from the frame grabber buffer fortransmission, and the entire process of sending audio and video to theDECspind application is repeated.

The third situation occurs when the DECspin application has not receivedan OK message within a specified "timeout" period 366. In a preferredembodiment, the timeout period is set at one half second. A timeout canoccur when, for instance, the network is reset, or a processor intensiveoperation is being performed at the receiving workstation which competeswith the DECspind application for workstation resources. In timeoutsituations, loss of part of the continuous audio signal as well as partof the video signal is unavoidable. In this case, the DECspinapplication continually trims 368 the audio buffer to only retain audiodata corresponding to the last time out interval, which preventsoverflow of the audio buffer. For instance, where the timeout intervalis one half second, DECspin only retains the last one half second ofaudio data in the audio buffer. Finally, when DECspin receives the OKmessage 370 from the DECspind application, DECspin first sends 364 thelast half second of digitized audio accumulated in the audio buffer, andthen proceeds 360 to determine 350 if a new frame of video data isavailable from the frame grabber buffer, and the entire process ofsending audio and video to the DECspind application is repeated.

FIG. 17 illustrates a more detailed timing analysis of these three videoframe acknowledgement situations. At time t₀, the frame grabber beginsto capture a digitized video frame in its buffer, which becomesavailable for transmission by the DECspin application at time t₁. Attime t₁, video FRAME1 is transferred from the frame grabber buffer tothe DECspin application buffer in main memory, and the frame grabberbegins to accumulate the next frame FRAME2, in the frame grabber buffer.

The DECspin application then begins to send the audio and video data tothe DECspind application shortly after time t₁. If the acknowledgement0K message for FRAME1 is received from DECspind before the next videoframe, FRAME2, is available at time t₂, i.e., during the ACK1 interval,then DECspin waits until time t₂ to begin sending the audio and videoFRAME2 data. If the acknowledgement OK message for FRAME1 is receivedfrom DECspind after FRAME2 is available at time t₂ but before the nextvideo frame, FRAME3, is available from the frame grabber buffer at timet₃, i.e., during the ACK2 interval, then DECspin begins immediatelysending the audio data accumulated in the audio buffer and video frameFRAME2 to the DECspind application.

In the case where the acknowledgement OK message is received greaterthan two frame times after video frame FRAME1 became available from theframe grabber buffer at time t₁, e.g., after time t₃ during the ACK4time interval, the currently available video frame is FRAME3 whichbecame available at time t₃. Since the frame grabber has only a singleframe buffer, the FRAME2 data is discarded, and not sent to the DECspindapplication. In this manner, only the freshest video frame is sentacross the network. Regardless of when the OK message is received afterthe two frame timeout limit, the last available (and freshest) videoframe is sent to the DECspind application, and all other interveningvideo frames are discarded. The last available video frame is sentshortly after the 0K message is received.

The DECspind application synchronizes the received audio data streamwith the video frame data to be displayed by comparing the audio datastream to the time stamps of the received video frame data. Due tosystem time delays and the order of audio and video data transmissions,a frame or video will be available for display by the DECspindapplication prior to reproduction of its corresponding audio. A typicalsystem time delay is about 1/4 to 1/2 second for the audio signal. Audiodata received by the DECspind application is accumulated into a DECspindaudio buffer, and read out for sonic reproduction on a continuous basis.DECspind keeps track of the number of audio data bytes received from theDECspin application, and thus generates a continuously updated audiotime stamp. As discussed above, each video frame is preceded by aVIDEOHEADER (FIG. 16) message which includes a timing information field428 holding a time stamp value corresponding to the audio byte count atthe end of that video frame time. DECspind reads this time stampinformation and delays displaying the associated reconstructed videoframe until an audio time stamp generated from the received audio datastream matches the video frame time stamp. The video frame is displayedwhen the match occurs. In this manner, the audio always appears to besynchronized to the displayed video frames, regardless of the videoframe rate or the loss of intervening video frames.

Graphical User Interface

FIG. 18 shows a graphical user interface (GUI) window 510 displayed onthe monitor of a workstation 12 (FIG. 2) for controlling a DECspin videoteleconferencing session. Teleconferencing session window 510 isgenerated by the DECspin application using X Windows managed by a Motifmanager, and is displayed as a graphic window on the color monitor 30 ofthe multimedia workstation 12 (FIG. 2). DECspin session window 510provides the user of a networked multimedia workstation with aninterface for accessing the top level video teleconferencing functionsof the DECspin application.

When a user invokes the DECspin application to begin a videoteleconference through, for instance, a UNIX command, the DECspinsession window 510 appears on the workstation color monitor. A windowtitle bar 511 identifies the DECspin application and version number tothe user. From this point on, all teleconferencing functions areaccessed through the graphical interface associated with this and otherMotif managed X Windows graphics. Furthermore, through this graphicalinterface, the user can also store and playback audio/video "filmclip"messages.

DECspin application window 510 includes a Motif menu bar 512 having aplurality of user selectable "pushbuttons" 514, 516, 518, 520, and 522.These "pushbuttons" are presented as a Motif RowColumn widget whichcontrols all the top level DECspin functions available to the user. Eachof the "pushbuttons" presented to the user through the Motif windows areactivated with a graphical pointing device such as a mouse. A"pushbutton" may for instance be activated by moving the mouse to causea graphical pointer to enter the area of the pushbutton to select thatbutton, and then activating a mechanical switch on the mouse to activatethe corresponding "pushbutton". Furthermore, the "pushbuttons" also lendthemselves to use with a touch screen display, where the user simplytouches the "pushbutton" on the screen to activate it. An activated"pushbutton" is highlighted to indicate activation.

The DECspin graphical user interface has no first or second levellanguage specific functional controls, and thus DECspin appears to theuser as an internationalized application. As such, all top levelfunction pushbuttons 514-522 of menu bar 512 are identified by iconicsymbols which have been internationalized where possible.

Activation of one of the Motif "pushbuttons" of menu bar 512, in mostcases, causes creation of a second level pop-up window. The second levelpop-up window in turn, offers the user next level of functional choicesassociated with the selected top level function.

An audio help, or "talking button" feature is also associated with theMotif "pushbuttons" available for selection by the user. Bysimultaneously selecting a Motif "pushbutton" while holding down akeyboard "help" key, a stored audio message is audibly reproduced whichdescribes the function of the selected "pushbutton".

A live video image appears in a video window 524 displayed below themenu bar 512. Video window 524 displays the current local image beinggenerated by the frame grabber 34 (FIG. 2) and stored in the video framebuffer 36 (i.e., video window 524 frames that section of the displaymemory frame buffer in which the video grabber stores each frame ofvideo.) The video window acts as a video "monitor" for viewing localvideo throughout the video teleconference session. Furthermore, the sizeof the video window 524 can be changed by using the mouse to move a sideor corner of the window. Changing the size of the video window affectsthe number of pixels encompassed by the window, and thus the number ofvideo pixels to be sent to another workstation during each video frame.Smaller windows achieve a higher frame rate since less data per framemust be sent. Conversely, larger windows achieve a lower frame rate.Thus, the video frame rate for a video conference connection can beadjusted by simply sizing video window 524.

When a called workstation invokes the "slave" DECspind application toreceive audio and video from a calling DECspin application, the DECspindapplication displays a DECspind session window 850 as shown in FIG. 19.The DECspind session window appears similar to the DECspin sessionwindow 510 of FIG. 18, except that the title bar 852 displays theidentification of the calling party, and the menu bar 854 can containonly an "exit" pushbutton 856. A video window 858 displays the videodata received from the DECspin application. Unlike the video window 524of the DECspin session window 510, video window 858 of the preferredembodiment is not resizable to change the video resolution since itssize depends on the resolution of the video data sent by the "Master"DECspin application. Furthermore, since the video teleconferencingparameters are set by the "Master" DECspin application, no functionalcontrols are provided other than an "exit" control which when activatedwill terminate the video teleconferencing connection. As an alternativepreferred embodiment, a "control" pushbutton can be added to theDECspind session window, which when activated offers the user a limitedcontrol set for adjusting the received video image or audible levels.Furthermore, a "monitor" pushbutton can also be added to allow the userto monitor, for instance, the data rate of the incoming videoteleconference associated with the DECspind session window.

Referring again to the DECspin application session window 510 of FIG.18, a functional description of the menu bar 512 "pushbuttons" is asfollows. The "exit" pushbutton 514 when activated forces all videoteleconferencing connections to the workstation to cease, and for theDECspin application to terminate. Any necessary cleanup of networkconnections is also done. If any changes to the DECspin configurationoccurred during the video teleconference session, a pop-up Motifquestion box (not shown) prompts the user to either save the changes,restore default settings, or quit. The icon which identifies the "exit"pushbutton 514 is the international symbol for "exit" (i.e., a greencircle with a vertical white bar on a white background). This symbol isused on all DECspin pop-up windows to indicate the pushbutton thatcauses the user to exit that particular pop-up window.

The "connections" pushbutton 516 is used to establish videoteleconference connections between workstations. When activated, thispushbutton causes creation of a second level "call list" pop-up window600 as shown in FIG. 20. Through the "call list" window the user canadd, delete, activate or modify network video teleconferencingconnections to other networked workstations. The user can also createand store an audio/video "filmclip" file through this pop-up window. The"connections" pushbutton icon is an international symbol showing a blackgrid with four intersection points on a white background.

The "control" pushbutton 518 is used to adjust various parametersassociated with the audio and video data generated by the workstationand sent to the other video teleconferencing participants. Activation ofthis pushbutton causes creation of a second level "control" pop-upwindow 650 as shown in FIG. 21. Through the "control" window the usercan adjust transmission parameters including the maximum video framesper second, video hue, video color saturation, video brightness, videocontrast, audio volume, and microphone gain. The user can also selectbetween video compression, on or off; color or black and white; and,transmission source, live or stored.

The "monitor" pushbutton 520 is used to view various teleconferencingand network system parameters. Activation of this pushbutton causescreation of a second level "monitor" pop-up window 700 as shown in FIG.22. Through the "monitor" window, the user can monitor the average videoframes per second transmission rate, the average network resourcesconsumed by the active DECspin video teleconference, the number of videoconference participants, the number of active audio and videoconnections, and the pixel resolution of the video image generated bythe local workstation.

The "help" pushbutton 522 is used to access DECspin on-linedocumentation. Activation of this pushbutton causes creation of a secondlevel "information" pop-up window 750 as shown in FIG. 23. Through this"help" window the user can access audio, video and text basedinformation, instruction, and help on various DECspin features, indexedby subtopics. The "help" pushbutton icon is a blue circle on a whitebackground having a white "i" in its center, which is the internationalsymbol for "information".

Referring to FIG. 20, the "call list" window 600 is activated byselecting the "connections" pushbutton 516 (FIG. 18) of the top levelDECspin session window. The "call list" window 600 is a Motif pop-upwindow which is made up mainly of four columns 602, 604, 606 and 608 ofMotif widgets. The widgets of the first column 602 include seven"connect" toggle pushbuttons 610a-610g each associated with a "networkhost" field 612a-612g, respectively, in the second column 604. Toestablish a connection with another workstation, the user enters thetarget workstation host name and user name into a "network host" fieldof column 604 and activates the corresponding "connect" togglepushbutton of column 602 to establish the connection. The syntax forthis "host network" field is given as "host:user" for TCP/IP, and as"host::user" for DECnet, respectively. If no user is specified, theDECspin application will attempt to contact anyone logged into thetargeted host workstation. The associated "connect" toggle pushbutton ofcolumn 602 is activated again to cause disconnection from the targetworkstation. A "connection" icon 614 located above the "connect" togglepushbuttons helps to indicate their function to the user.

The widgets of the third and fourth columns 606, 608 include seven"audio" toggle pushbuttons 616a-616g and seven "video" togglepushbuttons 618a-618g, each associated with a "network-host" field612a-612g, respectively. The "audio" toggle pushbuttons of 616a-616g ofcolumn 606 determine if the associated "network host" will receive audiofrom the workstation, and the "video" toggle pushbuttons 618a-618g ofcolumn 608 determine if the associated "network host" will receive videofrom the local workstation. An "audio" icon 620 indicates the functionof "audio" toggle pushbuttons 616a-616g, and a "video" icon 622indicates the function of "video" toggle pushbuttons 618a-618g. Thus,the user can establish a connection with another multimedia workstation(target host) by simply indicating the target host name in a "networkhost" field 612 and activating the associated "connect" togglepushbutton 610. Once the connection is established, the associated"audio" toggle pushbutton 616 determines if the multimedia workstationwill receive audio, and the associated "video" toggle pushbutton 618determines if the workstation will receive video.

Upon activation of a "connect" pushbutton of column 602, the DECspinapplication attempts to call the target host workstation over thenetwork. If a connection cannot be made, a Motif error box (not shown)appears on the local monitor and states what error has occurred. Theerror box may prompt the user for further instructions, such as retry orcancel.

"Call list" window 600 also includes a "join" pushbutton 624 which, whenactivated, joins all "network hosts" currently connected to the localworkstation into a single video teleconference. This produces an n-wayvideo teleconference between all the network hosts connected to theworkstation as indicated by an activated (highlighted) "connect" togglepushbutton 610a-610g.

The "call list" window 600 also allows the user to redirect audio andvideo to and from a multimedia file. An output file "connect" togglepushbutton 626 activates a connection to an output file as specified inan associated output file descriptor field 628. If an output fileconnection is activated while a teleconference is in progress, a copy ofthe outgoing transmission from the local workstation is stored to thespecified output file. An "audio" toggle pushbutton 630 and a "video"toggle pushbutton 632 associated with the output file descriptor field628 respectfully determine if audio, video, or both audio and video willbe stored to the output file. The audio and video data is stored to thedesignated output file until the output file "connect" toggle pushbuttonis deactivated.

An input file "connect" toggle pushbutton 634 activates playback of aninput file containing pre-recorded audio and/or video data, possiblystored earlier by means of the output file "connect" toggle pushbuttondiscussed above. The input file is specified in an input file descriptorfield 636 associated with the input file "connect" toggle pushbutton634. Furthermore, if the input file is selected for playback while avideo teleconference is in progress, the contents of the file are alsosent to the conference members as indicated by the active "connect"toggle pushbuttons 610a-610g. In this manner, an audio/video data streamcan be sent to and viewed by all conference members simultaneously.

A "filmclip" pushbutton 638 allows a user to view DECspin audio/videomessages stored as .SPN files in the conventional system message area,described below. Activation of this pushbutton causes a Motif "fileselection box" pop-up window (not shown) to appear which allows the userto select a message file for playback. The message is played back byrunning the stored audio and video data through a local DECspindapplication as if the data streams were being sent to the DECspindapplication by a remote DECspin application.

Finally, the "exit" pushbutton 640 of "call list" 600 terminates the"call list" pop-up window and returns the user to the activeteleconference windows established by means of the "call list" window.

When a call is placed to a target host by activating a "connect" togglepushbutton a "ring box" pop-up window 800 as shown in FIG. 25 appears onthe display of the remote workstation being called. Simultaneously, thekeyboard bell of the target host workstation is "rung". A DECspindapplication window 850, shown in FIG. 19, is also displayed on theremote workstation, but with inactive video. The "ring box" windowindicates to the user of the target host workstation that a videoteleconference call is being attempted with that workstation. The titlebar 802 0f the "ring box" identifies the calling host workstation andthe user who placed the call. No other information (including audio andvideo) is revealed to the called party unless the call is answered.

The "ring box" 800 offers the called party an "answer" pushbutton 804and a "busy" pushbutton 806. If the called party activates the "answer"pushbutton, the target workstation will use a local DECspin applicationto return audio and video to the calling workstation, and will displaythe received video in the DECspind application window 850 shown in FIG.19. The received audio is audibly reproduced by the DECspindapplication. This completes the two-way video teleconferencingconnection between the calling and the called workstations.

If the called party activates the "busy" pushbutton 806, the callingparty is informed that the called target workstation is busy.Furthermore, the called party can ignore the "ring box". In this case,if there is no answer after a set period of time, e.g., four rings, thecalling party is prompted to take further action, such as leave amessage or close the call. Still further, the called party can simply"hang up" by activating the "exit" pushbutton 856 of the DECspindapplication window of FIG. 19.

If a called party does not answer within a predetermined time period,answers busy, or just hangs up, the calling party is prompted with aMotif information box (not shown) which allows the caller to leave ashort audio/video message on the called workstation. This feature workslike an answering machine. The maximum length of the message is presetto a reasonable period of time, dependent on the file storage resourcesof the target system. In one preferred embodiment, the maximum messagelength is limited to 20 seconds, and is not changeable by the user. Theuser is presented with a pop-up Motif "countdown" box (not shown) whichprovides a running indication of the amount of time left while leaving amessage.

Messages may be stored in the conventional system message area with, forinstance, a .SPN file descriptor extension. Once the caller elects toleave an audio/video message, the DECspin application of the callingworkstation directs the audio and video data streams meant for thecalled workstation to a message file. Upon invocation of the DECspinapplication, the system message area is checked for any stored .SPNmessage files. If any are present, the user is presented with a pop-upMotif "file selection box" window (not shown) which allows the user toselect and playback any of the stored messages. The message is playedback by running the stored audio and video data streams through aDECspind application as if the data streams were being sent by an activeDECspin application. Message files may also be discarded through thispop-up window. Furthermore, the system message area can be checked forSON message files at any time by activating the "filmclip" pushbutton638 0f the "call list" window 600.

Referring to FIG. 21, the "control" pop-up window 650 is activated byselecting the top level "control" pushbutton 518 (FIG. 18) of theDECspin session window 510. The "control" window is a Motif pop-upwindow which is made up mainly of a column 652 of sliding scale widgetcontrols, and a column of corresponding icons 654 and 656 located oneither side of the sliding scale widgets to indicate the function of theassociated sliding scale widget. For instance, the top most slidingscale widget 652a adjusts the maximum video frame rate for the videoteleconference. The video frame rate sliding scale widget includes acontrol gadget 658a which may be moved with the mouse along the lengthof the sliding scale widget to adjust the maximum frame rate. A digitalreadout 660a above the sliding scale moves with the control gadget anddisplays the current frame rate setting. The left side icon 654a shows a"snail" to indicate movement of the control gadget to the left slows theframe rate. Conversely, the right side icon 656a shows a "hare" toindicate movement of the control gadget to the right increases the framerate.

The remaining sliding scale widgets 652b-652g operate in a similarmanner to effect their respective controls. Scale 652b-652g respectivelycontrol color hue (red on left, green on right), color saturation,brightness, contrast, audio volume, and microphone gain. Furthermore,"control" window 650 includes two pushbuttons 660 and 661 for enablingand disabling video compression, respectively. Two other pushbuttons 664and 666 enable black and white, or color video, respectively. Finally,an "exit" pushbutton 668 terminates the "control" window.

Referring to FIG. 22, the "monitor" pop-up window 700, activated byselecting the top level "monitor" pushbutton 520 (FIG. 18) of theDECspin application window 510. The "monitor" window is a Motif pop-upwindow which includes graphic indicators for displaying certain videoteleconferencing parameters and statistics. The displayed statistics areupdated periodically, for instance, every five seconds.

Information related to the video image generated by the DECspinapplication is displayed below a "camera" icon 702. The video imageinformation includes a digital readout 704 of the video image size inhorizontal by vertical pixels, and a sliding scale indicator 706 with anassociated digital readout 708 for showing the average frame rate. Thesetwo parameters are important since the DECspin video window 524 of FIG.18 may be "sized" as discussed above, to change the number of pixelscontained in the image. Changing the number of pixels also effects thevideo frame rate. Both these video parameters can be monitored by theseindicators as the DECspin window is sized.

Information related to the network and active video teleconference isdisplayed by indicators located below a "connection" icon 710. Thisinformation includes digital readouts of the total number of activeteleconferencing connections 712, the number of active audio connections714, and number of active video connections 716. Finally, a slidingindicator 718 and an associated digital readout 720 display the averagenetwork consumption. An "exit" pushbutton 722 terminates the "monitor"pop-up window.

Referring to FIG. 23, the "information" pop-up window 750 is activatedby selecting the "help" pushbutton 522 (FIG. 18) of the top levelDECspin session window 510 and allows the user access to textual and"filmclip" documentation. The "information" window is a second levelMotif pop-up window which is made up of a column 752 of user selectablepushbuttons configured as a Motif Radio Box so that only a singlepushbutton can be activated at one time. Each pushbutton is labeled witha sub-topic for which help documentation is available. For instance, inthe embodiment of FIG. 23, help documentation is available to provide anoverview of the video teleconferencing system 754, to help start aconference 756, to help with a specific video teleconferencing feature758, or to help troubleshoot a video teleconferencing problem 760. Uponactivation of one of the topic pushbuttons, a third level "sub-help"window 780 of FIG. 24 appears offering the user a further breakdown ofsubjects related to the selected topic. For instance, the "sub-help"window of FIG. 24 shows the subjects available to the user afterselecting the overview pushbutton 754 of FIG. 23.

The "sub-help" window 780 of FIG. 24 is a third level Motif pop-upwindow having a column of pushbuttons 782 and 784 on each side of acenter column 786 of subject labels. The left column of pushbuttons 782activates a bookreader textual documentation system for thecorresponding subject. For instance, activation of pushbutton 788 willpresent the user with text on the workstation display describing anintroduction to the video teleconferencing system. The right column ofpushbuttons 784 activates a "filmclip" for the corresponding subject.For instance, activation of pushbutton 790 will open another DECspindwindow on the display through which a "filmclip" demonstration will beplayed back to the user.

The help system thus described offers the user three levels ofcomprehensive textual, audio and visual system documentation. At thefirst level, the "talking keys" offer the user quick access to audioinformation about a function available for selection by the user. At thesecond level, the pop-up "information" window helps to narrow the user'shelp requirements to a specific topic. At the third level, the pop-up"sub-help" window offers the user textual and/or audio/visualdocumentation on a variety of subjects related to the selected secondlevel topic.

Furthermore, the file storage structure for the help documentation addsa high degree of flexibility to the audio/visual documentation system ofthis invention. For instance, each "audio clip" of the "talking key"help function is stored in a separate key-specific file which is playedback when the talking help function is activated. Thus, if the functionof a key changes, only a single audio file needs to be updated.Furthermore, the audio help system can be easily internationalized byproviding an "audio clip" file set corresponding to the desired foreignlanguage. This also applied to the textual and "filmclip" documentationwhich is stored in separate subject-specific files. Thus, if a systemfunction changes, only the text and/or "filmclip" files affected by thechange need to be updated. Although this audio/visual documentationsystem has been described with reference to the video teleconferencingapplication of this invention, it will be apparent to those skilled inthe art that such an audio and/or visual help system can be provided ona multi-media workstation or PC for virtually any type of application.

FIGS. 26(a)-26(k) show the display screens of three workstations,Workstation A (30a), Workstation B (30b), and Workstation C (30c) duringthe initiation of a typical three-way video teleconference using theGraphical User Interface of this invention. FIG. 26(a) shows a typicaldisplay 30a for Workstation A after the DECspin application has beeninvoked but before the initiation of a video teleconference. Thisdisplay shows a DECspin session window 510a for monitoring the localvideo generated by Workstation A. The user of Workstation A hasactivated the "call list" window 600a, the "control" window 650a, andthe "monitor" window 700a.

The user of Workstation A initiates a video teleconference call toWorkstation B ("host B") by activating pushbutton 610a of the "calllist" window. FIG. 26(b) shows the display 30b of Workstation B afterWorkstation A places the call. A "ring box" 800a and a blank DECspindsession window 850a, identifying the calling party ("host A") appear onthe Workstation B display. The user of Workstation B answers the videoteleconference call by activating the answer pushbutton of "ring box"800a.

FIG. 26(c) shows the display 30b of Workstation B after the user answersthe call from Workstation A. Here, the video (and audio) of the host ADECspind session window 850a has been activated. Further, a localDECspin session window 510b appears to allow the user of Workstation Bto monitor the local video signal. Here also, the user of Workstation Bhas invoked the "control" window 650b from the DECspin session Window510b. FIG. 26(d) shows the display of Workstation A after Workstation Bhas answered the video teleconference call. A host B DECspind window850b provides return video from Workstation B to Workstation A.

The user of Workstation A initiates a video teleconference call toanother user on Workstation C ("host C") by activating pushbutton 610bof the "call list" window 600a.

FIGS. 26(e) and 26(f) show the display 30c of Workstation C afterWorkstation A places the call, and after Workstation C answers the call,respectively. Here again, the user answering the call to Workstation Ccauses the invocation of a local DECspin session window 510c to monitorlocal video from Workstation C. FIG. 26(g) shows the display 30a ofWorkstation A after the user of Workstation C answers the call. A host CDECspind session widow 850c displays the return video from Workstation Cto Workstation A.

At this point the user of Workstation A can join the users ofWorkstation B and Workstation C into a three-way conference byactivating the "join" pushbutton 624 of the "call list" window 600a.Upon activation of the "join" pushbutton, Workstation B is caused toplace a video teleconference call to Workstation C, and vice versa.FIGS. 26(h) and 26(i) show the display of Workstation B and WorkstationC, respectively, after the call is placed, but before it is answered. A"ring box" and blank DECspind application session window fromWorkstation C (800c and 850c) appear on the display of Workstation B(FIG. 26(h)) and a "ring box" and blank DECspind application sessionwindow from Workstation B (800b and 850b) appear on the display ofWorkstation C (FIG. 26(i)).

FIGS. 26(j) and 26(k) show the display of Workstation B and WorkstationC, respectively, after the user of each workstation has answered theirrespective "ring box" of FIGS. 26(a) and 26(i). Thus, FIGS. 26(g), 26(j)and 26(k) show the resulting display of Workstations A, B, and C,respectively, when all are joined into a three-way video teleconference.

Equivalents

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

We claim:
 1. The video teleconferencing apparatus for a distributed dataprocessing system having a plurality of computer workstations connectedby a digital data network, the computer workstations comprising:a) amaster process executing on a processor of a local workstation formatingand sending audio and video data across the digital data network asdigital data packets to a remote workstation over a first variablebandwidth digital data connection; b) a slave process executing on aprocessor of the remote workstation receiving and reconstructing theaudio and video data from across the digital data network from themaster process of the local workstation for audible and visualreproduction, respectively; c) a master process executing on theprocessor of the remote workstation formatting and sending audio andvideo data across the digital data network as digital data packets tothe local workstation over a second variable bandwidth digital dataconnection; and d) a slave process executing on the processor of thelocal workstation receiving the audio and video data from across thedigital data network from the master process of the remote workstationfor audible and visual reproduction, respectively;wherein the masterprocess of the local workstation causes the execution of the slaveprocess on the remote workstation to receive the audio and video datasent by the master process of the local workstation, the slave processof the remote workstation causes the execution of the master process onthe remote workstation to send the audio and video data to the localworkstation, and the master process of the remote workstation causesexecution of the slave process on the local workstation to receive theaudio and video data sent by the master process of the remoteworkstation.
 2. The video teleconferencing apparatus of claim 1, whereinthe local workstation executes a slave process for each master processexecuting on remote workstations and sending audio and video data to thelocal workstation.
 3. The video teleconferencing apparatus of claim 1wherein the master process of the local workstation comprisesa) audiotransmission means for sending an audio data stream from the localworkstation to the remote workstation via the digital data network suchthat the audio data can be reconstructed by the remote workstation intoa continuous audio signal, and b) video transmission means for sendingvideo data from the local workstation to the remote workstation via thedigital data network such that each frame of video data sent to theremote workstation is inserted into the audio data stream by the localworkstation without affecting the continuity of the reconstructed audiosignal at the remote workstation.
 4. The video teleconferencingapparatus of claim 3 whereina) the video transmission means furthercomprises means for precluding a frame of video data from being sent tothe remote workstation in response to a system overload condition; andb) the audio transmission means further comprises means for sending theaudio data stream corresponding to the precluded frame of video data tothe remote workstation to prevent loss of continuity of the audio signalreconstructed by the remote workstation during a system overloadcondition.
 5. The video teleconferencing apparatus of claim 4 whereina)the slave process of the remote workstation comprises acknowledgementtransmission means for issuing a frame acknowledge message to the localworkstation for each frame of video data successfully received from thelocal workstation by the remote workstation; and b) the master processof the local workstation comprises acknowledgement receiver means forreceiving the acknowledgement message from the remote workstation andfor deferring the sending of another frame of video data from the localworkstation to the remote workstation until an acknowledgement messagefor the most recently sent frame of video data is received from theremote workstation.
 6. The video teleconferencing apparatus of claim 5wherein the system overload condition is determined by the elapsed timebetween sending a frame of video data to the remote workstation andreceiving an acknowledgement message for that frame of video data fromthe remote workstation.
 7. The video teleconferencing apparatus of claim6 whereina) the audio transmission means further comprises means forsending any audio data accumulated by the local workstation since thelast video frame was sent to the remote workstation immediately uponreceipt of the acknowledgement message from the remote workstation forthe last video frame if an overload condition exists; and b) the videotransmission means further comprises means for sending the most recentlyavailable frame of video data from the local workstation to the remoteworkstation and dropping any other intervening frames of video datawhich became available during the elapsed time of the overloadcondition; and c) the audio transmission means comprises means forsending to the remote workstation any audio data accumulated by thelocal workstation, since the last video frame was sent to the remoteworkstation, when another video frame becomes available at the localworkstation for sending to the remote workstation if no overloadcondition exists; and d) the video transmission means comprises meansfor sending the most recently available frame of video data from thelocal workstation to the remote workstation if no overload conditionexists.
 8. The video teleconferencing apparatus of claim 3 whereina) thevideo transmission means further comprises means for precluding a frameof video data from being sent to the remote workstation in response to asystem failure condition; b) the audio transmission means furthercomprises1) accumulating means for accumulating audio data at the localworkstation corresponding to a predetermined time interval; 2) means forsending the accumulated audio data to the remote workstation immediatelyafter the system failure condition is corrected; and c) the videotransmission means also further comprises means for sending the mostrecently available frame of video data to the remote workstation aftersending the accumulated audio data.
 9. The video teleconferencingapparatus of claim 3 whereina) the master process of the localworkstation further comprises means for attaching timing information toeach frame of video data sent to the remote workstation by the videotransmission means, the timing information indicating a point in thecontinuous audio data stream which corresponds in time to the frame ofvideo data; and b) the slave process of the remote workstation comprisessynchronization means for displaying a received frame of video on thedisplay of the remote workstation when the point in the audio streamcorresponding to the timing information of the received video frame isaudibly reproduced at the remote workstation.
 10. The videoteleconferencing apparatus of claim 9 whereina) the slave process of theremote workstation further comprises timing means for generating timinginformation by counting the amount of audio data received from the localworkstation in the continuous audio stream; and b) the synchronizationmeans further comprises comparison means for comparing the timinginformation generated by the timing means with the timing informationsent along with the most recently received video frame to determine whento display the most recently received frame of video data on the displayof the remote workstation.
 11. The video teleconferencing apparatus ofclaim 1 wherein the digital data connections comprise a TCP/IP protocoldata socket.
 12. The video teleconferencing apparatus of claim 1 whereinthe digital data connections comprise a DECnet protocol data socket. 13.The video teleconferencing apparatus of claim 1 wherein the digital dataconnections are established across an FDDI data network.
 14. The videoteleconferencing apparatus of claim 1 wherein the digital dataconnections are established across an ISDN data network.
 15. The videoteleconferencing apparatus of claim 1 wherein the digital dataconnections are established across an ethernet data network.
 16. Videoteleconferencing apparatus for a distributed data processing systemhaving a plurality of computer workstations connected by a digital datanetwork, the computer workstations comprising:a) source means for alocal workstation to send audio and video data across the digital datanetwork as digital data packets to a remote workstation, the sourcemeans comprising a master process executing on a processor of the localworkstation; b) receiver means for the local workstation to receiveaudio and video data from across the digital data network as digitaldata packets sent from source means of the remote workstation, thereceiver means comprising a slave process executing on a processor ofthe remote workstation; wherein the master process of a localworkstation causes execution of a slave process on a remote workstationfor receiving audio and video data sent by the master process of thelocal workstation over one variable bandwidth digital data connection,the slave process on the remote workstation causes execution of a masterprocess on the remote workstation for sending audio and video data tothe local workstation, and the master process of the remote workstationcauses execution of a slave process on the local workstation forreceiving audio and video data sent by the master process of the remoteworkstation over another variable bandwidth digital data connection. 17.The video teleconferencing apparatus of claim 16, whereina) the sourcemeans further comprises means for a local workstation to send audio andvideo data to a plurality of remote workstations; and b) the receivermeans further comprises means for the local workstation to receive audioand video data from the source means of the plurality of remoteworkstations; wherein the source means of the local workstation sendsaudio and video data to the receiver means of the plurality of remoteworkstations over a like plurality of variable bandwidth digital dataconnections, and the source means of each of the plurality of remoteworkstations sends audio and video data to the receiver means of thelocal workstation over another like plurality of variable bandwidthdigital data connections.
 18. The video teleconferencing apparatus ofclaim 17, wherein a local workstation executes a slave process for eachmaster process executing on a remote workstation sending audio and videodata to the local workstation.
 19. The video teleconferencing apparatusof claim 16 whereinthe source means comprises:a) audio transmissionmeans for sending an audio data stream from the local workstation to theremote workstation via the digital data network such that the audio datacan be reconstructed by the remote workstation into a continuous audiosignal, b) video transmission means for sending video data from thelocal workstation to the remote workstation via the digital data networksuch that each frame of video data sent to the remote workstation isinserted into the audio data stream by the local workstation withoutaffecting the continuity of the reconstructed audio signal at the remoteworkstation, and c) time stamping means for attaching timing informationto each frame of video data sent to the remote workstation by the videotransmission means, the timing information indicating a point in thecontinuous audio data stream which corresponds in time to the frame ofvideo data; and the receiver means comprises: d) synchronization meansfor displaying a received frame of video on the display of the remoteworkstation when the point in the audio stream corresponding to thetiming information of the received video frame is audibly reproduced atthe remote workstation.
 20. The video teleconferencing apparatus ofclaim 19 whereina) the receiver means further comprises timing means forgenerating timing information by counting the amount of audio datareceived from the local workstation in the continuous audio stream; andb) the synchronization means further comprises comparison means forcomparing the timing information generated by the timing means with thetiming information sent along with the most recently received videoframe to determine when to display the most recently received frame ofvideo data on the display of the remote workstation.
 21. The videoteleconferencing apparatus of claim 19 whereina) the video transmissionmeans further comprises means for precluding a frame of video data frombeing sent to the remote workstation in response to a system overloadcondition; and b) the audio transmission means further comprises meansfor sending the audio data stream corresponding to the precluded frameof video data to the remote workstation to prevent loss of continuity ofthe audio signal reconstructed by the remote workstation during a systemoverload condition.
 22. The video teleconferencing apparatus of claim 21whereina) the receiver means comprises acknowledgement transmissionmeans for issuing a frame acknowledge message to the local workstationfor each frame of video data successfully received from the localworkstation by the remote workstation; and b) the source means comprisesacknowledgement receiver means for receiving the acknowledgement messagefrom the remote workstation and for deferring the sending of anotherframe of video data from the local workstation to the remote workstationuntil an acknowledgement message for the most recently sent frame ofvideo data is received from the remote workstation.
 23. The videoteleconferencing apparatus of claim 22 wherein the system overloadcondition is determined by the elapsed time between sending a frame ofvideo data to the remote workstation and receiving an acknowledgementmessage for that frame of video data from the remote workstation. 24.The video teleconferencing apparatus of claim 23 whereina) the audiotransmission means further comprises means for sending any audio dataaccumulated by the local workstation since the last video frame was sentto the remote workstation immediately upon receipt of theacknowledgement message from the remote workstation for the last videoframe if an overload condition exists; and b) the video transmissionmeans further comprises means for sending the most recently availableframe of video data from the local workstation to the remote workstationand dropping any other intervening frames of video data which becameavailable during the elapsed time of the overload condition; and c) theaudio transmission means comprises means for sending to the remoteworkstation any audio data accumulated by the local workstation, sincethe last video frame was sent to the remote workstation, when anothervideo frame becomes available at the local workstation for sending tothe remote workstation if no overload condition exists; and d) the videotransmission means comprises means for sending the most recentlyavailable frame of video data from the local workstation to the remoteworkstation if no overload condition exists.
 25. The videoteleconferencing apparatus of claim 19 whereina) the video transmissionmeans further comprises means for precluding a frame of video data frombeing sent to the remote workstation in response to a system failurecondition; b) the audio transmission means further comprises1)accumulating means for accumulating audio data at the local workstationcorresponding to a predetermined time interval; 2) means for sending theaccumulated audio data to the remote workstation immediately after thesystem failure condition is corrected; and c) the video transmissionmeans also further comprises means for sending the most recentlyavailable frame of video data to the remote workstation after sendingthe accumulated audio data.
 26. The video teleconferencing apparatus ofclaim 16 wherein the digital data connections comprise a TCP/IP protocoldata socket.
 27. The video teleconferencing apparatus of claim 16wherein the digital data connections comprise a DECnet protocol datasocket.
 28. The video teleconferencing apparatus of claim 16 wherein thedigital data connections are established across an FDDI data network.29. The video teleconferencing apparatus of claim 16 wherein the digitaldata connections are established across an ISDN data network.
 30. Thevideo teleconferencing apparatus of claim 16 wherein the digital dataconnections are established across an ethernet data network.
 31. A videoteleconferencing computer workstation comprising:a video frame grabberfor generating local video data by digitizing analog video signalsreceived from a video source, and for accumulating said local video datato provide frames of said local video data; a video buffer for receivingand temporarily storing said frames of local video data; a networkcontroller for transmitting said frames of local video data temporarilystored by said video buffer to a remote computer workstation via a firstone-way variable bandwidth digital data connection providing a firstvirtual circuit connection across a digital data network; a centralprocessor for executing a local slave application program forconfiguring said video teleconferencing computer workstation to receiveframes of remote video data generated by said remote computerworkstation under control of a remote master application programcontrolling said local slave application program, said frames of saidremote video data being transmitted via a second one-way variablebandwidth digital data connection providing a second virtual circuitconnection across said digital data network, said central processorfurther executing a local master application program for establishingsaid first one-way variable bandwidth digital data connection to saidremote computer workstation and invoking a corresponding remote slaveapplication program in said remote computer workstation to configuresaid remote computer workstation to receive said frames of local videodata transmitted by said network controller.
 32. A videoteleconferencing computer workstation as claimed in claim 31, furthercomprised of said central processor, in response to sending a firstframe of said local video data and receiving an acknowledge signalindicative of receipt of said first frame by said remote computerworkstation only after a second frame of said local video data has beenaccumulated in said frame grabber, transmitting local audio data priorto transmitting said second frame.
 33. A video teleconferencing computerworkstation as claimed in claim 31, wherein said central processor, inresponse to sending a first frame of said local video data and receivingan acknowledge signal indicative of receipt of said first frame by saidremote computer workstation only after a plurality of frames of saidlocal video data have been accumulated in said frame grabber,transmitting only a most recent one of said plurality of frames.
 34. Avideo teleconferencing computer workstation as claimed in claim 31,wherein said digital data network comprises a fiber distributed datainterface.
 35. A video teleconferencing computer workstationcomprising:a video frame grabbar for generating local video data bydigitizing analog video signals received from a video source, and foraccumulating said local video data to provide frames of said local videodata; a video buffer for receiving and temporarily storing said framesof local video data; a network controller for transmitting said framesof local video data temporarily stored by said video buffer to a remotecomputer workstation via a first one-way variable bandwidth digital dataconnection providing a first virtual circuit connection across a digitaldata network; and a central processor for executing a local masterapplication program establishing said first one-way variable bandwidthdigital data connection to said remote computer workstation and invokinga corresponding remote slave application program in said remote computerworkstation to enable said remote computer workstation to receive saidframes of local video data transmitted by said network controller;wherein said remote slave application program running on said centralprocessor of said remote computer workstation invokes a remote masterapplication for controlling generation of frames of remote video data,said remote master application program signalling said videoteleconferencing computer workstation to enable a local slaveapplication program for receiving said frames of remote video data. 36.A video teleconferencing computer workstation as claimed in claim 35,wherein said remote master application is invoked in response to aremote user of said remote workstation sanctioning a videoteleconference between said video teleconferencing computer workstationand said remote workstation.
 37. A two-way video teleconferencingconnection between a local workstation and a remote workstation across adigital data network, said two-way video teleconferencing connectioncomprising:a first one-way connection between a local-master applicationof said local workstation and a remote-slave application of said remoteworkstation for providing locally-generated video data generated by saidlocal workstation to said remote workstation for display by said remoteworkstation; and a second one-way connection between a remote-masterapplication of said remote workstation and a local-slave application ofsaid local workstation for providing remotely-generated video datagenerated by said remote workstation to said local workstation fordisplay by said local workstation.
 38. A two-way video teleconferencingconnection as claimed in claim 37, wherein said first one-way connectionand said second one-way connection are different variable bandwidthdigital data connections across said digital data network.
 39. A videoteleconferencing connection between a plurality of workstations across adigital data network for providing audio and video data, said videoteleconferencing connection comprising a multiplicity of one-wayconnections from master applications running on each of saidworkstations to corresponding slave applications running on every otherone of said workstations so that every one of said slave applications ofeach of said workstations is dedicated to communication with a singlemaster application of a different one of said workstations.
 40. A videoteleconferencing connection as claimed in claim 39, wherein said one-wayconnections are different variable bandwidth digital data connectionsacross said digital data network.
 41. A method for establishing andmaintaining video teleconferencing link between a local computerworkstation and a remote computer workstation, said methodcomprising:executing a local master application program by said localcomputer workstation establishing a first one-way variable bandwidthdigital data connection providing a first virtual circuit connectionacross a digital data network to said remote computer workstation; andinvoking a corresponding remote slave application program by said localmaster application in said remote computer workstation to enable saidremote computer workstation to receive frames of local video datagenerated by said local computer workstation; executing a local slaveapplication program in said local computer workstation by said remotecomputer workstation for configuring said local computer workstation toreceive frames of remote video data generated by said remote computerworkstation under control of a remote master application programcontrolling said local slave application program, said frames of remotevideo data being transmitted via a second one-way variable bandwidthdigital data connection providing a second virtual circuit connectionacross said digital data network.
 42. A method as claimed in claim 41,further comprising:generating said local video data by digitizing analogvideo signals received from a video source at said local computerworkstation, and for accumulating said local video data to provide saidframes of local video data; and transmitting said frames of said localvideo data to said remote slave application program of said remotecomputer workstation via said first one-way variable bandwidth digitaldata connection.
 43. A method as claimed in claim 41, furthercomprising, in response to sending a first frame of local video data andreceiving an acknowledge signal indicative of receipt of said firstframe by said remote computer workstation only after a second frame ofsaid local video data has been accumulated, transmitting local audiodata prior to transmitting said second frame.
 44. A method as claimed inclaim 41, further comprising, in response to sending a first frame oflocal video data and receiving an acknowledge signal indicative ofreceipt of said first frame by said remote computer workstation onlyafter a plurality of frames of said local video data have beenaccumulated, transmitting only a most recent one of said plurality offrames.
 45. A method for establishing and maintaining videoteleconferencing link between a local computer workstation and a remotecomputer workstation, said method comprising:executing a local masterapplication program by said local computer workstation establishing afirst one-way variable bandwidth digital data connection providing afirst virtual circuit connection across a digital data network to saidremote computer workstation; invoking a corresponding remote slaveapplication program by said local master application in said remotecomputer workstation to enable said remote computer workstation toreceive frames of local video data generated by said local computerworkstation; and invoking a remote master application in said remotecomputer workstation by said remote slave application for controllinggeneration of frames of remote video data, said remote masterapplication signalling said local computer workstation to enable a localslave application program for receiving said frames of remote videodata.
 46. A method for establishing and maintaining videoteleconferencing link between a local computer workstation and a remotecomputer workstation, said method comprising:executing a local masterapplication program by said local computer workstation estabilishing afirst one-way variable bandwidth digital data connection providing afirst virtual circuit connection across a digital data network to saidremote computer workstation; invoking a corresponding remote slaveapplication program by said local master application in said remotecomputer workstation to enable said remote computer workstation toreceive frames of local video data generated by said local computerworkstation; invoking a remote master application in said remotecomputer workstation by said remote slave application for controllinggeneration of frames of remote video data, said remote masterapplication signalling said local computer workstation to enable a localslave application program for receiving said frames of remote video databeing transmitted via a second one-way variable bandwidth digital dataconnection providing a second virtual circuit connection across saiddigital data network; generating said remote video data by digitizinganalog video signals received from a video source at said remotecomputer workstation, and for accumulating said remote video data toprovide said frames of remote video data; and transmitting said framesof remote video data to said local slave application program of saidlocal computer workstation via said second one-way variable bandwidthdigital data connection.